"I know, it's not the sexiest of themes, but the proof of its impact is undeniable," Gates wrote. "The lives of the poorest have improved more rapidly in the last 15 years than ever before."

The letter describes remarkable progress from schools in the United States to health care in Ethiopia and makes the case that continued investments in these efforts have made a measurable difference for millions of the world's poorest people. A core reason was the world's commitment to setting clear goals and identifying the right measures to drive progress towards those goals.

This is particularly true in the case of foreign aid. "Historically, aid was largely discussed in terms of the total amount of money invested. Now that we're more precisely measuring indicators like child mortality, people are able to see the impact aid has in stark terms—that it's the difference between putting people on AIDS treatment or letting them die," Gates states in the letter.

He goes on to emphasize the importance of this, given tight budgets around the world. "Not only do clear goals and measures allow governments to spend their aid money more efficiently, it builds the political will to continue funding aid programs by proving how successful they are. It's not just about governments giving other governments taxpayer money; it's about one community helping another raise itself out of poverty."

Gates emphasizes the importance of the United Nations' Millennium Development Goals (MDGs) as an example of how the world is capable of achieving really big things when we come around a clear goal and develop the measurements to gauge progress. The MDGs are a set of eight specific goals that are an unprecedented global effort to meet the needs of the world's poorest by 2015. "While we won't reach all of the goals, the progress we've made toward each is staggering," said Gates. "The MDG target of reducing extreme poverty by half has been reached ahead of the deadline, as has the goal of halving the proportion of people who lack access to safe drinking water."